Various hydraulic systems have been used for post tensioning or to lift heavy loads such as preformed concrete walls, etc. Many of these types of systems have used steel strand which passed through chucks. For example, see U.S. Pat. No. 3,658,296 issued Apr. 25, 1972 in the name of Lawrence R. Yegge; No. 3,837,621 issued Sept. 24, 1974 in the name of Juan Bautista Ripoll Gomez; No. 2,833,118 issued May 6, 1958 in the name of L. Nixon; and No. 4,106,752 issued Aug. 15, 1978 in the name of Eduardo Caro Roqueta.
Generally, these previous systems have had the disadvantage that they were designed for post tensioning concrete reinforcement members, not for heavy lifting and lowering of a load. It was very difficult to lift and lower a load any substantial distance since there was no easy way to engage and disengage the sets of chucks.
In accordance with the preferred embodiment of the present invention, a plurality of steel strands are passed through upper and lower sets of chucks which each comprise a plurality of jaws held together by a compression spring which biases the chuck assembly in a disengaged position from a chuck chamber. The chuck chambers are threaded members which are screwed into upper and lower plates. A seating ram is used to seat the chucks within the chuck chamber whenever it is desired to grasp and hold the steel strand passing through them. Items are lifted by seating the upper chucks and then extending the pistons of the rams to effect the lifting operation. Once the pistons are fully extended, further lifting is accomplished by seating the lower chucks, releasing the upper chucks and bringing the pistons back to their original position. The upper chucks are then reseated and the pistons of the rams are extended. This procedure is repeated until the load is moved the desired distance. Relief space about the chucks, an angular slope to their top flange, combined with their multiple part construction enhance their releasability from the chuck chambers and minimize drag on the steel strand when the chucks are not seated and the strand is pulled through in either direction. A hydraulic system ensures the smooth lifting or lowering of the load.